Both alliance AND horde have shadow resistant races (undead and draenei), so it is useful for PvP shadowpriests and warlocks.

For PvE - not a very good stat.

Comment by Nilez

What you are saying is total BS, penetration is a very good stat for pve, about every mob in an endgame raid has some resistance to every type, about every boss aswell, so having about 40-60 penetration doesn't hurt

Comment by Overhead

Mobs in PvE resist due to their level, not magical resistances. On the vast majority of PvE mobs, spell penetration is useless. To get rid of those 'resist' messages, you need spell hit, not spell penetration.